1. Field of the Invention
The present invention relates to using nuclear magnetic resonance to measure properties of an earth formation. Specifically, the measuring is performed in a borehole.
2. Description of the Related Art
In exploration for hydrocarbons, it is important to make accurate measurements of geologic formations. The geologic formations below the surface of the earth may contain reservoirs of oil and gas. The geologic formations may include formation layers and various structures. In a quest for oil and gas, it is important to know about the location and composition of the formation bedding planes and the various structures. In particular, it is important to know about the geologic formations with a high degree of accuracy so that resources are not wasted. Measuring properties of the geologic formations provides information that can be useful for locating the reservoirs of oil and gas. Typically, the oil and gas are retrieved by drilling boreholes into the subsurface of the earth. The boreholes also provide access for taking measurements of the geologic formations.
Well logging is a technique used to take measurements of the geologic formations from the boreholes. In one embodiment, a “logging instrument” is lowered on the end of a wireline into the borehole. The logging tool sends data via the wireline to the surface for recording. Output from the logging instrument comes in various forms and may be referred to as a “log.” One type of measurement involves using nuclear magnetic resonance (NMR) to measure properties of the geologic formations.
A recent generation of nuclear magnetic resonance (NMR) logging instruments based on using permanent magnets was introduced about a decade ago (see, for example, U.S. Pat. No. 4,717,878 issued to Taicher et al., U.S. Pat. No. 5,055,787 issued to Kleiberg et al., and U.S. Pat. No. 6,452,388 issued to Reiderman et al.). The recent generation of NMR logging instruments demonstrated advantages over previous NMR technologies such as earth's magnetic field NMR logging. Advantages provided included higher signal-to-noise-ratio (SNR), higher resolution in acquiring NMR relaxation spectra, capability of diffusion measurements, and a defined volume of investigation with no effects of protons in a borehole fluid on acquired NMR data.
The recent generation of NMR logging instruments typically exhibited a high rate of decreasing a static magnetic field of the permanent magnet with distance from an NMR sensor. The high rate of decreasing the static magnetic field generally results in a relatively small region of investigation. As a consequence of the small region of investigation, the SNR of NMR measurements is too low to allow for a desired logging speed with acceptable vertical resolution. The low SNR also limits the NMR measurements to a depth of investigation of about two to four inches. The depth of investigation of about two to four inches is a region substantially invaded by drill mud spurt. In general, the drill mud spurt interferes with talking accurate NMR measurements of the geologic formations.
What are needed are apparatus and methods for performing the NMR measurements with a higher SNR than previously achieved in a logging instrument. Preferably, the higher SNR allows for the desired logging speed with acceptable vertical resolution and for the region of investigation exceeding two to four inches of depth.